Heat a pan over medium high heat, and cook the bacon until crispy. Remove the bacon to a work bowl and reserve for later.
Add the slices mushrooms to the pan, and cook until the mushrooms are browned, and most of the moisture has evaporated. If more oil is required, add some vegetable oil as needed. This should take 10 to 15 minutes depending on the heat used.
Move the mushrooms from the pan to a work bowl, and reduce the heat.
Add the spinach to the pan, and cook, stirring frequently, until the spinach is wilted. Adding vegetable oil as needed.
Once the spinach is wilted, move to a colander and allow to cool.
Mixing The Frittata
Crack the eggs into a large work bowl, and whisk with a fork, or a whisk until the eggs are smooth and uniform looking.
Chop up the bacon into small pieces, and mix in to the eggs.
Chop up the mushrooms into small pieces, and add them to the egg mixture. Mix well.
Squeeze out as much moisture as you can out of the spinach, and then chop up the spinach roughly, and mix in to the egg mixture. The more moisture you can remove, the better the texture of the final frittata will be.
Mix in the crumbled feta cheese, and half of the Parmesan cheese. Stir to combine as well as you can.
Building The Frittata
Use the butter to grease the inside of a 9×13 inch casserole dish.
Pour the mixture in to the casserole dish, and flatten out, spreading out the ingredients as required to get a flat even appearing top.
If your casserole dish is much larger, you may need to add more eggs. To do so, crack two eggs into a work bowl, and mix, and add them to the casserole dish and stir to combine.
Repeat until the casserole dish is sufficiently full, and there are no holes in the mixture. I fill the dish to about half to three-quarters full.
Cover the casserole dish in foil, and refrigerate until ready to cook.
Cooking the Frittata
Pre-heat the oven to 350 degrees.
Add the foil covered casserole dish to the oven, and cook for 45 minutes, until the mixture is firm, and mostly solid.
Remove the foil, and top the frittata with the remaining shredded Parmesan cheese, and cook for an additional 15 minutes, or until the cheese has browned.
Remove, from oven and allow to cool for a few minutes, slice and serve.
Recipe Notes
Cooking time will vary, if you want a eggier texture (similar to an omelette), the cook the Frittata covered in foil for less time, likely 30 minutes rather than 45 minutes.